Up until recently, dams forming lakes and reservoirs typically fell within two general categories of construction. These categories are rock and earth fill dams and conventional poured mass concrete dams.
Earth or rock filled dams are less expensive than conventional mass concrete dams. Earth and rock filled dams, however, suffer substantial disadvantages. For example, earth and rock filled dams cannot withstand "over topping" of the water, as this obviously causes erosion of the dam structure. Thus, earth and rock filled dams must be filled higher than otherwise would be required, dramatically boosting the volume of the fill, and leading to a dam of excessively thick cross section and larger aerial extent. A separate spillway must be constructed to release excess water, particularly during flood conditions. The end result is that this type of dam structure, especially for impounding larger bodies of water, is relatively expensive to build.
In addition, earth and rock filled dams are prone to leakage as the water tends to weep through interstices in the structure. As a result, grouting procedures and other stop-gap devices, such as continuous plastic liner, must be utilized to make the dam retain enough water to keep the lake full. Periodic regrouting and other leakage preventive maintenance must be carried out. As can be readily seen, the overall cost of this type of dam is thus further increased, rather dramatically.
Conventional poured mass concrete dams advantageously are of narrower cross section and of less aerial extent. Also, leakage is not as great a problem. However, using poured concrete for the entire structure with the attendant forms and finish work required is considerably more expensive and time consuming.
Recently, dams are being constructed of roller compacted concrete (RCC), a damp gravel fill blended with cement and compacted by rolling in layers behind a precast concrete panel assembly. Advantageously, unlike rock fill dams, RCC dams can withstand over topping during flood conditions. Further, they can be built with the same narrow cross section and low aerial extent as mass concrete dams.
RCC dam also has many advantages over conventional poured mass concrete dams. Specifically, roller compacted concrete mixing requirements are less stringent requiring both less concrete and allowing a wider range of aggregates to be used in the mix. Thus, local aggregates often can be used at cost savings not available with conventional poured concrete. Roller compacted concrete dams also require less labor than conventional mass concrete dams as there is relatively little forming and the concrete is spread by highway scrapers rather than bucket by bucket. Advantageously, these characteristics result in roller compacted concrete dams of comparable strength costing as much as one third less and built in as much as one third the time as conventional poured mass concrete dams.
Despite all of these advantages, prior art roller compacted concrete dams still suffer from some problems that must be addressed and solved. Specifically, the roller compacted concrete is spread in substantially horizontal layers to form the dam. This typically creates a weak plane along the horizontal cold joints between layers that can eventually allow the passage of water. As the water leaks in these planes, the enormous static head pressure can actually provide sufficient lifting force to cause parts of the dam to uplift or slide. Further, during winter months, the water trapped between the roller compacted concrete layers near the surface of the dam may freeze and expand placing further pressure on the cold joints and possibly increasing the leak flow rate over time. Eventually, this could result in an unstable condition and the need for costly dam repairs.
It, therefore, is clear that a need exists for an improved dam structure and method of construction appreciably limiting or eliminating water leakage through the roller compacted concrete dam.